Hybrids!

It's very uncommon that monsters of different species impregnate each other but it can happen. When two monsters of different species mate, there's a 10% chance one will be impregnated and give birth to a hybrid monster. 20% if they're a related monster species.

In those cases that do happen, the hybrid is born with a combination of traits, some looking like one species of monster only to have one subtle trait left over from the other parent or literally look comparable to that of a Chimera. With that comes a mixture of natures though they're personality will be more so dependent on which parent the hybrid is around. They'll also potentially inherit powers from both sides and potentially outshine many other monsters.

They're isn't a stigma against them unless they're born from two species who have a hatred of each other, in which case they'll receive prejudice from their parent's species usually. Every other monster will just see them as a monster.

It's a little big planet so who knows what types of hybrids will pop up... along with the potential fun along with them.

Important note: If a hybrid continues to try to breed further with another, unrelated non hybrid species, than the chances of impregnation drop down to 5%. 1% for hybrid on hybrid breedings. Those breedings can produce highly rare hybrids of peak power but after that, If a two time hybrid mates with something else, they also have the chance to make their parent hybrid. For example, should a Pheonix fuck a Bunny girl, there is a possibility that a Mountain Roc or a Dragon could be born, or if a Cerberus fucked a Elf, a Hellhound or Hydra could be born. But even then, the chances of impregnation are low, around 2%.